On weight of photon

hi pf, my friends asked me a strange question to me today,
"if in a vacuum chamber if light is somehow focused on a suitable weighing machine then will there be any deflection?
if yes does that mean light has mass? "

Staff: Mentor

Focused in which way? Coming from above, reflecting light upwards? This will transfer momentum, and the scale will change.
Focused to have a high and constant energy density close to some (hypothetical) very sensitive scale? The scale should see an attraction.
Something else?

Staff: Mentor

Light has a momentum, and momentum times intensity is a pressure. Pressure times surface area (of your scale, or focus) is a force, and the scale can measure this.
The Nichols radiometer is not a scale, but it operates with light pressure.